1. Field of the Invention
This invention relates to capillaries and the design thereof for use in conjunction with wire bonding to the bond pads of semiconductor devices and a method of forming the bond.
2. Brief Description of the Prior Art
Ball bonding is a widely used technique in semiconductor fabrication to connect the internal semiconductor die to the external leads. In this procedure, a fine gold wire, usually about 25 .mu.m (0.0010 inch) to about 30 .mu.m (0.0013 inch) is fed down through a ceramic capillary, generally alumina, having an entry aperture at the top and an exit aperture at the opposite end of a bore therein. A ball is formed external to the exit aperture by an electronic flame off (EFO) mechanism that melts a small portion of the wire remaining after the previous bond. Essentially, the ball is formed at the end of the wire by an electric discharge spark. At this time, the capillary is relatively far from the ball (millimeters distant). The wire is restrained from moving by a tensioner until the ball is centered in the chamfer diameter of the capillary and is forced downward by the continued motion of the capillary toward the bond pad on the die. The ball is placed on a bond pad of the semiconductor device being assembled and the capillary end then forces the ball against the pad to provide the bond in conjunction with thermosonic energy.
The above described ball bonding step presents a major obstacle to gold wire ball bonding in integrated circuits with the bond pads closer than approximately 100 .mu.m (0.0039 inch) due to the diameter of the capillary. As the pad-to-pad pitch of semiconductor devices decreases, there is less room for the capillary to make a ball and bond the ball and attached wire to a pad without interfering with the ball and wire on an adjacent pad. Current fine-pitch gold ball bonding uses a fine pitch or "bottlenose" capillary that allows finer pitch bonding than is possible with a standard capillary. However, it is not possible to shrink the capillary diameter sufficiently to produce bond pitches below approximately 90 .mu.m (0.0035 inch) without causing stitch bond strength degradation. The reason for this stitch bond strength degradation is that the dimensions of the face of the capillary that forms the stitch bond shrink as the capillary diameter shrinks, thereby reducing the area in which the stitch is formed. Also, great stress is placed on the capillary bond face during the stitch bond. The capillary bond tip is literally forced into the leadframe, leaving an imprint of the capillary tip in the lead finger, thereby presenting the same problem as discussed above with regard to the ball bond. Additionally, capillary cost increases due to reduced manufacturing yields and capillary life is reduced because the capillary with reduced diameter is more fragile than a standard capillary. It is therefore apparent that an improved capillary or an improved technique for making wire bonds to pads of semiconductor devices will be increasingly desirable as the dimensions and spacing of the bond pads shrink.